So far only small numbers of insects have been found, but it is only a matter of time before the Western corn rootworm becomes a problem for maize farmers in Germany. In North America, and in many countries of Southern and Eastern Europe, the beetle is spreading rapidly. Once it has become established it is not easy to control. In the spring numerous bee colonies died in south-west Germany because of incorrect treatment of the maize seed. The authorities had ordered the measure in order to halt any further spread of the Western corn rootworm (Diabrotica virgifera).
Between July and September, the peak flight period of the Western corn rootworm , pheromone traps are set up in many of the German maize-growing areas. Following the first discoveries in 2007 near Freiburg, large numbers of the beetles were caught in July this year in Bavaria, Baden-Württemberg and neighbouring Alsace. To combat a rapid spread of the new pest in southern Baden and large areas of Germany, the authorities immediately introduced control measures. As well as insecticide treatment against the adult beetles, infestation and safety zones were set up around the spots where the insects had been found. Depending on the number of beetles observed, the farmers have to comply with certain conditions, the most severe of these requiring them to cease growing maize as part of the crop rotation for one or two years.
If Diabrotica spreads in Germany, the Julius Kühn Institute in Braunschweig estimates that around 350 000 of the 1.8 million hectares now under maize cultivation would be at risk. In the absence of control measures, they estimate that the damage would amount to at least EUR 25 million per year.
Diabrotica is a major maize pest in the USA. The costs for the damage it causes there and for controlling the pest amount to around one billion US dollars each year. Around the world, an area of around 20 million hectares is infested with the Western corn rootworm, 13.5 million of them in the USA alone. Diabrotica is the pest against which most insecticide applications are targeted. Each year insecticides to control the Western corn rootworm are used on 5.2 million hectares.
Poorly treated – bee deaths from insecticide treatment of maize
In 2008, in order to prevent the pest spreading further or even becoming established in Germany, the whole maize-growing area in the infestation and safety zones was sown only with maize seed that had been treated with an insecticide containing a particular active ingredient to protect it against the Diabrotica larvae. This was in response to a recommendation from the expert authorities that had advised increasing the application volume as a precautionary measure.
In late April and early May 2008 bee deaths were recorded in some regions of south-west Germany – in precisely those areas where the treated seed had been sown – which, according to the latest figures, resulted in around 11 000 bee colonies being affected, some of them seriously. Research by the Julius Kühn Institute (JKI) and other expert authorities confirm that the active ingredient Clothianidin used in the seed treatment was responsible for the bee deaths. As the result of incorrect treatment of the maize seed, dust was produced during sowing which meant that the insecticide was carried to flowering plants. It had long been known that Clothianidin is harmful to bees, however it had been assumed that there would be no contact between bees and the active insecticide ingredient applied to the seed.
As a precaution, in mid-May the German Federal Office of Consumer Protection and Food Safety (BVL) therefore ordered the suspension of the approval for various active ingredients used in seed treatments. It will not be decided until the autumn at the earliest whether seed treatments with the active ingredient Clothianidin will be available to maize farmers next year. It is possible that improved procedures for applying the treatment and sowing the seed will be prescribed in order to rule out the production of dust to a large extent.
Genetically modified maize:
The weapon of choice against the Western corn rootworm in the USA
Since 2003 genetically modified maize varieties have been available on the American market that express a special variant of the Bt toxin which specifically targets the larvae of the Western corn rootworm, in a similar way to the familiar concept used to combat the European corn borer . Numerous varieties are now available that offer a combination of resistances against both pests as well as herbicide tolerance . In 2007, between 40 and 45 per cent of the maize plants grown already had this kind of ’stacked’ resistance.
In particular, the new GM lines are proving very successful against the beetle larvae, which seriously damage the maize roots. The adult beetles eat the leaves and above all the maize silks, which reduces kernel formation. This type of damage is less important in the USA.
In Europe first release trials with Diabrotica-resistant lines are taking place, including in Spain, Eastern Europe and Germany. However, these varieties are not likely to be introduced onto the market in Europe in the next few years.
Maize cultivation: Preparing for Diabrotica
In Germany so far the only sightings have been of isolated beetles that have flown in. The urgent measures taken by the authorities, such as insecticide spraying against the beetles, the creation of zones with short-term bans on maize cultivation, and the recommendations regarding special treatment of the maize seed are designed to prevent the beetles becoming established and building up a resident population.
According to experts, however, the spread of the pest in Europe cannot be prevented, so maize farmers will have to prepare for it and will need appropriate control strategies in the near future. The US experience has shown that Diabrotica-resistant maize varieties can be an effective means of combating the larvae on the plant roots. While these GM varieties are not approved for cultivation in Europe, treating the maize seed with insecticides or applying soil insecticides are alternative options.
Breaking up the maize crop rotation with cereals or leaf crops would interrupt the main food supply of the Diabrotica beetle, but on its own this kind of measure will only reduce the pest pressure and the harvest losses. It cannot eradicate the pest.
